Believe It Or Not, Science Still Can't Explain Gravity

Brilliance | Dec. 12, 2017

Of all the forces in the universe, gravity is the one we're most intimately familiar with. Every time we roll out of bed, lift a coffee cup, or even just stand there, we feel its effects. So it may surprise you to learn that of all the forces in the universe, gravity is also the most difficult to explain scientifically. To understand why, you'll have to take a dive into the realms of relativity and quantum mechanics.

Gravity Wants To Bring Me Down

Gravity is one of the four fundamental forces of the Standard Model. There's a classic xkcd comic where one character tries to explain these forces to another character, beginning by easily explaining gravity and electromagnetism. He starts getting tripped up around the strong nuclear force, and once he gets to the weak nuclear force, he can only mumble incoherently. In the comic's alt text, he says "Of these four fundamental forces, there's one we don't really understand." The other character begins to reply, "Is it the weak force or the strong—" "It's gravity."

That pretty well explains the bizarre nature of what we do and don't understand about the universe. We can identify what matter consists of down to the subatomic level, and how the weak nuclear force and the strong nuclear force holds those particles together. We know that the electromagnetic force binds charged particles.

But gravity? Gravity is a whole other ballgame. For one thing, it's much weaker than the other three forces. The electromagnetic force pushing two positively charged protons apart, for example, is 10<sup>36</sup> times stronger than gravity's force pulling them together. And while every other force has an opposite — a push and a pull — gravity only seems to pull. This garden-variety force is the most mysterious one we've got.

That's not to say we don't understand gravity in practical terms. Newton's laws of motion work perfectly well for falling apples and speeding trains. The problem is that they fall apart when things get too small or too big, making it hard to understand things like atoms and black holes.

Relativity vs. Quantum Mechanics: The Ultimate Showdown

The problem with gravity comes down to a conflict between the two best rules we have for how the universe behaves: Einstein's general theory of relativity, and quantum mechanics. See, quantum mechanics is simple, relatively speaking. It focuses on the interactions between individual things, and assumes that the passage of time and the shape of space are fixed, for the most part. But relativity says that all of those interactions happen within the fabric of spacetime, which can bend and stretch depending on the things doing the interacting. So when you look at the force of gravity between, say, a photon and an electron, it's a lot to take in: you have to account for not only every possible quantum interaction between them, but also all possible configurations of the underlying spacetime.

Clever scientists have come up with ways around this. The theoretical particle called the graviton is one fix, but so far our most powerful detectors haven't found evidence for their existence. Others say gravity could be a fancy form of quantum entanglement. But perhaps the most likely explanation for why we can't explain gravity the way we can explain the other three forces is that it's not an individual force, but something that naturally emerges from all the other parts of the universe. The same way a game arises from bits on a computer chip or a dramatic play arises from the work of cast members, gravity may just be the end result of all the little things that happen all around us.